Extrusive igneous rock is a rock
that is formed from lava.
This rock forms as a volcano
erupts,
and the temperatures get suddenly
much cooler than they were in the Earth's interior.
When the
temperatures get cooler, minerals
crystallise and form solid rock from
lava.
The same happens when lava leaks out of a mid-ocean ridge, or
erupts from a submarine lava and comes into contact with water, in a
process where the melt is extruded
from the Earth's interior.
By Bill Shupp via Flickr.com
There are a few types of environments for this to happen. Under the
water, pillow lavas form.
By Bill Shupp via Flickr.com On
the ground, there can be different types of volcanoes. It
can be
a
calm, slow lava flow, or it can be a very explosive volcano; and it can
be anything between.
What type of volcano it is, depends on temperature and magma's
composition, more exactly its silica content.
By aprilandrandy via Flickr.com
If
the magma contains little
silica
and more ferromagnesian minerals
instead, it is mafic, hot, and contains much volatiles, it has low
viscosity and will not erupt violently but it forms a lava flow.
By Bill Shupp via Flickr.com
Either
glassy or crystalline rocks such as basalt
are formed. Many Hawaiian
volcanoes are of that type.
By Greg Bishop via Flickr.com
If the magma contains
a lot of silica,
it is felsic,
it is relatively cool, and contains
little volatiles, the eruption is very
explosive.
By ser_is_snarkish via Flickr.com
It will blow its top
off, spread volcanic debris and ash to the sky, and fragmented rocks
like tuff
are formed.
Mount St Helens was one of those explosive
stratovolcanoes.
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English, which is the English we use in
Australia.
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